Chelators

Metals are often insoluble in seawater, and if added may rapidly form insoluble hydroxides and not be available for algal uptake. Various chelators (organic chemicals) are available which bind with the free metal ions forming a metal complex which may be utilized by microalgae. Ethylenediaminetetraacetic acid (EDTA) is commonly used because it is not readily metabolized by bacteria. The di-sodium salt, Na₂EDTA.2H₂O has improved solubility characteristics and is the most widely used. However when making trace metal solutions where EDTA is required it is paramount to completely dissolve the EDTA before adding further trace metals as precipitation can still be a problem. Citrate (or citric acid) has been recommended because of its lower affinities for metals commonly found in seawater (Ca, Mg). Thus citrate should produce a more constant degree of chelation in situations where salinity fluctuates. A limitation of citrate is its high biodegradability which also provides organic material for bacterial growth. The theoretical chelator:metal ratio should be 1:1 but as some chelator will be bound to Ca and Mg in the seawater, more chelator should be added to ensure all metals stay in solution (ratios ranging up to 3:1; McLaughlin 1973).